1. Field of the Invention
The present invention relates to methods of transmitting and signaling over a reverse link in wireless communication systems.
2. Description of Related Art
Wireless third generation (3G) communication systems are currently introducing technologies in order to become spectrally efficient, while supporting data services, so as to provide improved multiplexing voice and multiplexing data services, for example. These efforts have resulted in the development of the 1×EV-DO and 1×EV-DV standards, an evolution of the CDMA2000 standard from the 3GPP2 body of standards. Similarly, the Universal Mobile Telecommunication System (UMTS) standard has introduced several advanced technologies, or enhancements as part of the High Speed Downlink Packet Access (HSDPA) specification. An aspect in all of these enabling technologies is to ensure that any associated control information is carried in an efficient manner.
The next generation of wireless services may be dominated by wireless data services, as opposed to current generation systems where voice telephony is the dominant service. Wireless standards are evolving to be more efficient for transport of data services by introducing several enhancements. Much of the standardization involving these enhancements to date has focused on the forward link (also known as the downlink, from base station (or Node B) to the mobile station (or user equipment (UE)). The enhancements include (i) fast scheduling to exploit channel variations; (ii) adaptive data rate selection; and (iii) Hybrid Automatic Repeat Request (H-ARQ).
Since wireless channels are typically characterized by rapid fading, scheduling of user transmissions when they experience an “up-fade” (a temporary surge in the channel quality due to fading), may greatly enhance capacity. When scheduled during an up fade, either the power can be lowered for the same data rate or, for the same power, a much higher data rate may be achieved. For data services, the latter is preferred, and may be achieved through adaptive data rate selection via adaptive modulation and coding (use of higher data rates for higher magnitude fades and lower data rates for lower magnitude fades). However, due to latencies and measurement errors, perfect estimation of the fade value (or equivalently the achievable signal-to-noise ratio) may not possible; thus the data rate selection process may be inherently error-prone and could result in high transmission error rates on the channel.
H-ARQ is a technique that lends considerable robustness to the process of adaptive rate selection. H-ARQ essentially ensures that no transmission is “wasted”. If a data block is transmitted and is received in error, the data block can subsequently be retransmitted. H-ARQ provides the ability to combine the previous transmission with the retransmission and then decode the combined transmission(s), so that energy from the erroneous transmissions is not wasted.
The reverse link (UE to Node B), also known as the uplink, has several important differences to the downlink. Firstly, the uplink is a “many-transmitters-one receiver” link as opposed to the downlink, which is a “One-transmitter-many receivers” link. In the uplink direction the controlling entity may not have all the information. Secondly, in the uplink of CDMA systems, soft handoff, where a UE can communicate simultaneously with several Node Bs, when the UE is in the coverage area of those Node Bs, is freely performed. By contrast, there is a cost associated with enabling soft handoff on the forward link, since the added Node B transmitter has to be activated to transmit to the user and has to expend additional power. Thirdly, a total received power normalized by the thermal noise (also called Rise Over Thermal or RoT) is a constraint on the uplink, as RoT should be below a certain threshold. Scheduling algorithms and/or rate selection algorithms must work within this constraint. These differences give rise to many challenges in the application of the above enhancements to the uplink.